This invention relates generally to coatings for electrical conductors and, more particularly, to such coatings that provide for low friction to reduce connector insertion forces. This invention also relates to methods for making connectors incorporating such coatings.
Conventional electrical connectors, such as automotive connectors having terminals made of tin-plated copper alloys, are designed so that a normal (i.e. compressive) force acts on the contacting surfaces of the connectors when they are in service. This force provides for low contact resistance leading to good electrical conductivity through the connector. A consequence of this normal force is that a significant insertion force is required to mate the male and female connector pair. The amount of insertion force required per terminal imposes a limitation on the number of terminals or circuits that can be manually connected simultaneously, because of the limit to the amount of force that can be applied. For example, a typical automotive connector is specified to receive a maximum of about 75 Newtons of insertion force. For a terminal made from traditional tin (Sn)-plated copper (Cu) alloy having a cross-section of about 0.4 mm2, this maximum imposes a practical limitation of about 30 to 40 terminal pairs in a manually-mated connector.
Due to the general increase in the number and complexity of electronic components and systems used in the automotive, computer, and telecommunications fields, a need exists for increasing the number of circuits per connector without significantly increasing the mating force required to join these additional circuits. Additionally, this increase in the number of components leads to a need for reducing connector size, and therefore terminal cross-section area. However, a reduction in connector size can lead to an increase in the occurrence of damages to connectors, such as from terminal bending or breakage, upon connector insertion.
Two basic approaches have been used previously to reduce or eliminate the insertion forces discussed above. The first approach seeks to solve this insertion force problem through use of mechanical designs, such as zero-insertion force (ZIF) connector designs. However, ZIF connectors are not routinely used in automotive or other demanding services, due to their complexity, cost, and inability to meet the environmental requirements of these services.
The second approach seeks to obtain low-insertion force (LIF) connectors by use of lubricants or low-friction materials. A number of lubrication methods have been devised to reduce insertion force by coating the terminal to produce a low-friction surface. One method has involved coating connectors or terminals with liquid lubricants to reduce necessary insertion force. However, these liquid lubricants can yield hazardous vapors, especially during their application. They also can migrate, evaporate, or gather dust and contamination. This can lead to degraded electrical and mechanical performance of the connector and associated electrical components over extended periods of time.
In view of the difficulties described above, use of thin film coatings of solid lubricants is considered to be better than use of liquid lubricants. Solid coatings solve the insertion force problem by providing the contact surfaces of electrical connectors with low coefficients of friction (COF) and low contact resistance (CR). Low COF minimizes insertion force and low CR reduces electrical losses, as well as prevents damage to connectors from excessive heating of the contact area due to low electrical conductivity. Specifically, these solid thin films preferably should provide COF levels of lower than 0.2, more preferably lower than 0.1, and most preferably lower than 0.08 to solve the high insertion force problem. At the contact surfaces of the terminals, the coatings also preferably should provide CR levels of less than 20 milliohms, more preferably less than 10 milliohms, and most preferably less than 8 milliohms. This solid coating should be made of a sufficiently wear-resistant material to withstand the wear and tear caused by multiple coupling and decoupling of the connectors, as well as by vibration present in automotive environments.
The most common materials used in manufacturing electrical connectors are copper and copper alloys, due to their high electrical conductivity and low cost. However, copper can easily oxidize, and its conductivity can thereby quickly degrade with time. Coating of copper with inexpensive electroplating materials, such as tin (Sn), lead (Pb), and tin-lead alloys, is known to prevent this oxidation. However, copper coated with these conventional electroplating materials typically has an unacceptably high COF level higher than 0.4. Tin-plated copper also cannot withstand the wear and tear caused by multiple terminal engagements, due to its softness. Such materials therefore are not suitable for use on LIF terminals. For example, U.S. Pat. No. 5,849,424 to Sugawara et al. entitled “Hard Coated Copper Alloys, Process for Production Thereof and Connector Terminals Made Therefrom” proposes numerous terminal coatings essentially consisting of copper, tin, nickel (Ni), and phosphorous (P). Another approach described in U.S. Pat. No. 4,925,394 to Hayashi et al. entitled “Ceramic-Coated Terminal for Electrical Connection” discloses a ceramic coating that has greater wear resistance than the uncoated copper terminal. Although such coatings can have sufficient hardness to withstand the wear and tear caused by multiple couplings, and have low CR, they do not have sufficiently low COF levels for use in the LIF applications discussed above. Ceramic coatings are so hard that they cause rapid wear of stamping dies used to make electrical terminals from coated copper strips, and they also may crack under the force of terminal mating.
Coatings of noble metals, such as gold (Au), silver (Ag), or platinum (Pt), also have been applied to copper terminals to prevent their oxidation. Although noble metals have very low CR levels, they are too expensive for use in common electrical connectors. Furthermore, noble metals also have a relatively high COF. For gold coatings engaged against gold coatings, the self-COF (i.e. gold-on-gold) level reaches values higher than 1.0. Although silver (Ag) coatings are less expensive than gold, they also generally are characterized by high self-COF values in the range of 0.8 to 1.2 for bulk silver, and 0.2 to 0.3 for silver coatings incorporating microcrystalline structures. Furthermore, silver coatings are not highly resistant to corrosion. As a result, silver coatings are unsuitable for prolonged operation, especially in corrosive environments.
The above oxidation-resistant but high-COF coatings, or their alloys, are combined with solid lubricants, such as graphite, and various plastics, such as polyamide, polyimide, and particularly polytetrafluoroethylene (PTFE), to form composite coatings having decreased COF levels. To increase wear resistance, materials such as nickel or titanium (Ti) also are added into these composite coatings. For example, U. S. Pat. No. 5,316,507 to Capp entitled “Noble Metal and Solid-phase Lubricant Composition and Electrically Conductive Interconnector” discloses several composites of noble metals incorporating graphite particles. The noble metal is selected from gold, silver, platinum, palladium, or alloys of these. The noble metal content of these coatings is higher than 95 weight percent. U.S. Pat. No. 5,679,471 to Cheng et al. entitled “Silver-Nickel Nano-composite Coating for Terminals of Separable Electrical Connectors” describes a coating having low CR and improved wear resistance. The COF of one such coating, Ag81Ni19, is 0.5 at 2 Newtons load, with a CR of about 20 milliohms. These coatings should have silver content preferably higher than 40 atomic percent, and most preferably higher than 73 atomic percent, to achieve the desired high electrical conductivity and wear resistance. U.S. Pat. No. 5,967,860 to Ricketts et al. entitled “Electroplated Ag—Ni—C Electrical Contacts” discloses electrodeposited silver-nickel-carbon coatings having CR levels in the range of 1 milliohm to 10 milliohms. The silver content of these coatings is higher than 60 atomic percent. Although some compositions of noble metal-solid lubricant composite coatings mentioned above have both low COF and low CR levels, their high noble metal contents make them too expensive for use in common electrical connectors. Therefore, they are economical only for use in special, high reliability applications.
U.S. Pat. No. 6,007,390 to Cheng et al. entitled “Low Friction Metal-Ceramic Composite Coatings for Electrical Contacts” discloses ceramic-metal composite coatings for electrical contacts. The composite coatings of silver or gold with titanium nitride (TiN), co-deposited by physical vapor deposition, have CR levels varying in the range of 1.9 milliohms to 4.4 milliohms at 5 Newtons load. The COF of TiN-silver on titanium level is higher than 0.2 at 1 Newton load. The noble metal content of these composite coatings varies between 1 and 10 atomic percent. Although these coatings are economically more attractive than the noble metal composite coatings discussed above, due to their lower noble metal contents, their COF levels are higher than those required for LIF connectors.
U.S. Pat. No. 5,028,492 to Guenin entitled “Composite Coating for Electrical Connectors” describes metal-polymer particle composite coatings prepared by plating technique. A metal is selected from tin, lead, tin-lead, tin-indium, or tin-silver alloys. The polymer is selected from polyimide, polyamide, or PTFE. The polymer content of these composite coatings is kept below 0.7 weight percent to provide high electrical conductivity. The initial CR of these composite coatings varies in the range of 2 milliohms to 10 milliohms. The frictional force of these coatings varies in the range of 3 gmf to 25 gmf U.S. Pat. Nos. 5,667,659 entitled “Low Friction Solder Electrodeposits” and 5,853,557 entitled “Low Friction, Ductile, Multilayer Electrodeposits,” both to Souza et al, describe composite coatings of tin, lead, or tin-lead alloys incorporating PTFE particles, which are prepared using an electrodeposition process. These coatings have COF levels in the range of 0.06 to 0.75 at 100 grams load, and CR levels in the range of 1.2 milliohms to 2.6 milliohms. The polymer particle content of such composite coatings should be very low, preferably lower than 1 weight percent to provide low CR values. Controlling the particle content of these coatings at such low levels by using the polymer particle metal composite plating processes mentioned above is difficult.
Molybdenum disulfide (MoS2) also is known as a solid lubricant in the form of dry powder, a solid coating, or a thin film, or in a mixture with greases. MoS2 is known to contribute to very low COF surfaces in bearings and cutting tools. Pure molybdenum sulfide (i.e. MoSx where x≦2) coatings are known to yield COF levels as low as 0.01 at 5 Newtons load. However, a moist environment quickly deteriorates the lubricating properties of pure molybdenum sulfide coatings causing the COF to increase above 0.15. To overcome this environmental limitation, MoS2-metal composites have been developed, as taught, for example, by B. C. Stupp in Thin Solid Films, vol. 84, pp. 257-266 (1981) entitled “Synergistic Effects of Metals Co-Sputtered with MoS2”. Stupp discloses composites of MoS2 and a variety of metals and metal alloys, including aluminum (Al), chromium (Cr), cobalt (Co), molybdenum (Mo), nickel, platinum, silver, gold, tantalum (Ta), tungsten (W), brass, and bronze. Stupp also teaches that chromium, cobalt, nickel, and tantalum are preferred metals for forming composites with MoS2. U.S. Pat. No. 6,423,419 to D. Teer et al., entitled “Molybdenum-sulphur coatings” cites the use of numerous MoS2-metal composites, including those of MoS2 and titanium, zirconium (Zr), hafnium (Hf), tungsten, niobium (Nb), platinum, vanadium (V), tantalum, chromium, molybdenum, and gold. Likewise, other MoS2 composites have been taught by Stupp and by M. C. Simmonds, et al. in Surface and Coatings Technology vol. 126, pp 15-24 (2000) entitled “Mechanical and tribological performance of MoS2 co-sputtered deposits”. These composites include MoS2 combined with the metals such as silver, aluminum, gold, iron (Fe), platinum, lead, nickel, or copper, or also with compounds such as antimony oxide (Sb2O3) and tungsten selenide (WSe2). All of these MoS2-metal composites are for use in tooling or bearings, where low friction and high wear resistance in humid environments is required.
None of the above publications disclose a MoSx-metal composite coating having a low CR level that is suitable for electrical connectors. In its pure form, MoS2 does not exhibit the electrical conductivity levels required for use in electrical conductors. As for the composite coatings incorporating MoS2 particles, U.S. Pat. No. 5,028,492 to Guenin, entitled “Composite Coating for Electrical Connectors,” explains that a tin-PTFE composite performs better than a particle composite of 99 weight percent tin and 1 weight percent MoS2 in the fretting test.
It should be appreciated from the foregoing description that there is a need for an improved coating for electrical contacts exhibiting high electrical conductivity (i.e., low CR), and low insertion force (i.e., low COF). The present invention fulfills this need and provides further related advantages.